The invention described herein is particularly useful in an adaptor for attaching a self-contained camera, with an automatic exposure control, to an image-forming optical system such as a microscope. An important consideration in adapting such a camera for photography through a microscope or other instrument is to preserve the automatic exposure control feature without reducing the amount of light available for the photographic exposure of a film unit within the camera.
Prior art arrangements for coupling self-contained cameras with automatically controlled exposure systems to optical instruments propose the diversion of some part of the useful light, light intended for the photographic image, onto the photosensitive detector in the automatic exposure control of the camera. Typically, this diversion is accomplished by a beam splitter or similar optical element within the instrument.
U.S. Pat. Nos. 3,130,634 & 3,292,490 exemplify the foregoing scheme. The former shows a conventional arrangement with a beam splitter inside the phototube to deflect a large fraction of light to an exposure control system. As with other conventional systems, the diversion markedly reduces the available light at the film plane. The latter patent shows a camera attached to make a photograph through one eyepiece of a binocular microscope and a fiber optic light pipe connecting the other eyepiece of the binocular microscope to the photodetector of the camera's automatic exposure control. The optical arrangement of such a microscope uses an internal beam splitter to divert half of the available light to each eyepiece. It makes light available to actuate the automatic exposure control attached to one eyepiece, but only at the expense of the photograph made through the other eyepiece. This intentional reduction in the illumination of the image, merely to actuate the automatic exposure control, can have a critical and deleterious effect on a photograph made through an image-forming optical instrument because the amount of light available is generally quite limited.
The prior art also includes a photographic microscope system such as the Zeiss ULTRAPHOT II. The photographic portion of the system is integrated with the microscope objective optics to form a special camera. The camera is neither self-contained nor functional apart from its place in the system. It has a photodetector at the focal plane and outside the boundary of a mask defining its photographic format. The illumination intensity at the focal plane is the least of any location along the optical path. Thus, the focal plane is an inefficient place to locate the detector for an exposure control. Moreover, at the focal plane a detector is most likely to give a spurious indication of the required exposure due to an anomaly in the magnified image.